1. Field of the Invention
The present invention relates to a positive electrode for a lithium-sulfur battery and a lithium-sulfur battery, and more particularly, to a positive electrode for a lithium-sulfur battery which can provide a lithium-sulfur battery exhibiting effective energy density and long cycle life characteristics.
2. Description of the Related Art
The development of portable electronic devices has led to a corresponding increase in the demand for secondary batteries having both a lighter weight and a higher capacity. To satisfy these demands, the most promising approach is a lithium-sulfur battery with a positive electrode made of sulfur-based compounds.
With respect to specific density, the lithium-sulfur battery is the most attractive among the currently developing batteries since lithium has a specific capacity of 3,830 mAh/g, and sulfur has a specific capacity of 1,675 mAh/g. Further, the sulfur-based compounds are less costly than other materials and are environmentally friendly.
Lithium-sulfur batteries use sulfur-based compounds with sulfur-sulfur bonds as a positive active material, and a lithium metal or a carbon-based compound as a negative active material. The carbon-based compound is one which can reversibly intercalate or deintercalate metal ions, such as lithium ions. Upon discharging (i.e., electrochemical reduction), the sulfur-sulfur bonds are cleaved, resulting in a decrease in the oxidation number of sulfur (S). Upon recharging (i.e., electrochemical oxidation), the sulfur-sulfur bonds are reformed, resulting in an increase in the oxidation number of the S. The electrical energy is stored in the battery as chemical energy during charging, and it is converted back to electrical energy during discharging.
Sulfur reacts with lithium metal to convert lithium sulfide or lithium polysulfide in the lithium-sulfur battery. The theoretical final oxidation state of sulfur is S8 and the final reduction state is Li2S. When S8 is completely reduced to Li2S (100% utilization), the lithium-sulfur battery exhibits the nominal capacity pf 1675 mAh/g, which is the highest energy density of all batteries.
A positive active material composition is produced by mixing a positive active material, a binder and a conductive agent in a solvent to prepare a positive active material composition, and coating the composition on a current collector. However, it is easy for the positive active material, the conductive agent, and the binder to coagulate in the solvent, thus causing a phase separation. Such a phenomenon causes an uneven surface to form on the positive electrode, thus decreasing the reactive surface area, which deteriorates energy density and cycle life characteristics.